The present disclosure relates in general to semiconductor manufacturing technology, and more particularly, to process control of mask making in the semiconductor manufacturing technology. The present disclosure also relates to a method for controlling phase angle of a phase-shift mask by post-treatment in the semiconductor manufacturing process.
In semiconductor manufacturing technology, phase-shift masks (PSM) have been used to overcome problems associated with light diffraction through small openings patterned on the reticle. One example of PSMs is attenuated PSMs, which form their pattern through adjacent areas of quartz and molybdenum silicide (MoSi). Unlike chrome, MoSi allows small percentage of light to pass through the small openings. The thickness of MoSi is chosen so that the light passing through MoSi is 180 degrees out of phase with the light passing through the clear quartz areas. Even though the light passing through MoSi areas is too weak to expose the resist, the phase difference pushes the intensity down to be darker than similar features in chrome. As a result, a sharper intensity profile is provided, which allows smaller features to be printed on the wafer.
Phase shift or phase angle is expressed as a function of thickness of MoSi and quartz. The phase angle becomes larger as the thicknesses of the MoSi and quartz increase. In order to control the thickness of MoSi, the etching time of MoSi is monitored. In a traditional mask making process, after a patterned chrome mask is formed with a chromium plasma shield, a first etching is performed with SF6O2 plasma. A second etching or an over-etching is then performed with SF6O2 plasma on the patterned mask to control the critical dimension uniformity of the mask. After over-etching is completed, the chromium plasma shield is stripped and the circuit layout is formed on the PSM. However, once the PSM is complete, there is no way to adjust the phase angle in subsequent processing. Thus, if the phase angle of the PSM is out of specification because of the etching process, the whole PSM mask must be scrapped. In addition, subsequent mask cleaning process may also cause the MoSi loss, which leads to phase angle loss.
Therefore, a need exists for a method that controls phase angle of the PSM after all processes are completed, such that less PSMs will be scrapped due to processing defects.